Disclosed in JP2006-151362A is a brake apparatus for a vehicle that executes an anti-skid control (ABS control) by using a pressure regulating reservoir (a switch reservoir). Illustrated in FIG. 12 is a cross-sectional diagram of the pressure regulating reservoir disclosed in JP2006-151362A.
The pressure regulating reservoir illustrated in FIG. 12 is configured so that a major diameter portion 101a of a shaft 101, which is press-fitted into a piston 100, contacts a lower surface 102a of a valve seat 102, and so that an upper surface 100a of the piston 100 does not contact an upper end surface 105 of a wall surface defining a reservoir chamber 104 within a housing 103. Therefore, a lifting degree (lifting) of a ball valve 106 may be determined on the basis of only a size of an upper portion of the shaft 101 other than the major diameter portion 101a thereof, which contacts the lower surface 102a of the valve seat 102, i.e. on the basis of only a size of a minor diameter portion 101b of the shaft 101 in an axial direction thereof.
According to the pressure regulating reservoir disclosed in JP2006-151362A, the shaft 101, which is press-fitted into the piston 100, is upwardly pushed in FIG. 12 by an elastic force generated by a spring 109 while a normal brake operation is performed (e.g. while a brake hydraulic pressure control such as an anti-skid control (ABS control) and the like is not executed). Accordingly, the ball valve 106 is disengaged from the valve seat 102, thereby establishing a valve opened state. When the brake hydraulic pressure control is executed subsequently and a predetermined amount of a brake fluid flows into the reservoir chamber 104, the shaft 101 is downwardly displaced in FIG. 12 together with the piston 100, therefore, the ball valve 106 contacts the valve seat 102. Accordingly, the brake fluid may be prevented from flowing into the reservoir chamber 104 in order to prevent the piston 100 from reaching a bottom dead point (i.e. a bottom end point in a distance where the piston 100 is displaceable).
However, according to the pressure regulating reservoir disclosed in JP2006-151362A, when a brake hydraulic pressure is applied to a reservoir port 108, i.e. an upper portion of the pressure regulating reservoir relative to the valve seat 102 in FIG. 12, the shaft 101, which contacts the ball valve 106, and the piston 100 are pushed down, so that the pressure regulating reservoir turns to be in a valve closed state from the valve opened state. Therefore, generally, a certain amount of the brake fluid needs to be applied to the pressure regulating reservoir in order to turn the pressure regulating reservoir to be in the valve closed state when the brake operation is performed, while the brake fluid is also supplied to wheel cylinders. Therefore, a brake feeling may deteriorate because the brake fluid is also supplied to the pressure regulating reservoir in addition to the wheel cylinders.
Disclosed in JP2008-7080A is a pressure regulating reservoir, which is configured as a normally closed valve, in order to prevent a brake fluid from flowing into a reservoir chamber from a master cylinder when a brake operation is started, while ensuring a pressure regulating function of the pressure regulating reservoir.
Generally, there exists a need for reducing a deterioration of the brake feeling, which is caused because the brake fluids is also used at the pressure regulating reservoir, and for improving a responsiveness of the brake hydraulic pressure control.
For example, referring to FIG. 12, a balance between the brake hydraulic pressure and an internal pressure of the reservoir chamber 104 (which will be hereinafter referred to as a reservoir internal pressure) is determined on the basis of a relationship between a force for pressing down the piston 100 together with the shaft 101 (i.e. a seat diameter (an internal diameter of the valve seat 102)* the brake hydraulic pressure) and a force for pressing up the piston 100 together with the shaft 101 (i.e. the reservoir internal pressure (which corresponds to an intake negative pressure generated by a pump)* a piston diameter). In order to turn the pressure regulating reservoir to be in the valve opened state while the normal barking operation is performed, a relationship “reservoir internal pressure*piston diameter>seat diameter*brake hydraulic pressure” needs to be satisfied. Therefore, the seat diameter is limited to be enlarged. However, in order to flow the brake fluid into the reservoir chamber 104 by a suction of the pump so that the ball valve 106 and the valve seat 102 are pushed up to a point where the lifting degree of the ball valve 106 and the seat valve 102 of the pressure regulating reservoir reaches the maximum (i.e. a valve fully-opened state), an intake diameter of the pressure regulating reservoir needs to be enlarged. Furthermore, in order to improve the responsiveness of the brake hydraulic pressure control especially when the brake operation is performed under a cold temperature circumstance such as a circumstance where viscosity of the brake fluid increases, a size of the pressure regulating reservoir needs to be enlarged.
A need thus exists to provide a pressure regulating reservoir which is not susceptible to the drawback mentioned above.